Modern petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the wellbore, along with data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging”, can be performed by several methods. In conventional wireline logging, the drill string is removed from the wellbore and a probe containing selected instrumentation is lowered into the wellbore on a wire that supports the instruments and provides a direct communication link to the surface.
It is often desired to collect data during the drilling process while the drill string is in the wellbore, thus allowing the driller to make accurate modifications or corrections as needed to optimize performance. Designs for measuring wellbore conditions and formation parameter during drilling have come to be known as “measurement-while-drilling” (MWD) or “logging while drilling” (LWD) techniques. While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term MWD will be used with the understanding that this term encompasses both the collection of formation parameters and the collection of information relating to the movement and position of the drilling assembly.
Data gathered by MWD tools is typically transmitted to the surface by sensors or transducers located at the lower end of the drill string. While drilling is in progress, these sensors continuously or intermittently monitor selected drilling parameters and formation data and use some form of telemetry to transmit the information to a detector located at the surface. There are a number of telemetry systems in the art that seek to transmit information regarding downhole parameters up to the surface without requiring the use of a physical connection, such as a wire.
Acoustic telemetry is one of the systems used for MWD applications and operates by creating acoustic signals that travel to the surface along the drill string or through the fluid in the well. The acoustic telemetry signal is received at or near the surface using an accelerometer, or some similar device that is sensitive to motion. When acoustic telemetry is attempted through the drill pipe, each pipe joint acts as a reflector of acoustic radiation. The net effect of all these reflections is the creation of a comb-like structure of pass bands and stop bands where the range of frequencies of any of the pass bands is fairly narrow. The combined effect of a narrow frequency band and multiple reflections causes nodes and anti-nodes to be distributed along the pipe, where the signal is strongest at a node and weakest at an anti-node.
The transmission of data can be negatively affected by pulse spreading, distortion, attenuation, modulation rate limitations, and other disruptive forces, such as the ambient noise in the drill string. The largest source of ambient noise in the drill string is the drilling rig. Thus, receivers located at or near the surface are subjected to high levels of ambient noise generated by the drilling rig. These high levels of noise can interfere with the reception of telemetry signals and often require additional filtering or other processing of the received signals before useful analysis can be performed.
Therefore, while receiving telemetry signals at the surface is an important aspect of MWD or LWD processes, the inherent noise of the drilling rig often makes the surface one of the least desirable locations for placing a receiver. Accordingly, there remains a need to develop telemetry signal receiving methods and apparatus that overcome certain of the foregoing difficulties while providing more advantageous overall results.